This application relates to a refrigerant system wherein a main expansion device such as a thermostatic or electronic expansion valve is provided with a bypass line having an auxiliary expansion device such as a fixed orifice, capillary tube or accurator. The bypass line is selectively closed or opened dependent upon the amount of refrigerant flowing through the refrigerant system such that the smaller main expansion device can be used to handle lower amounts of refrigerant typically circulating throughout the system at normal operating conditions, and the auxiliary expansion device positioned on the bypass line is only utilized when higher refrigerant flows need to be accommodated.
Refrigerant systems are known in the art, and typically circulate a refrigerant to condition a secondary fluid such as air. As an example, in a basic air conditioning system a compressor compresses a refrigerant and delivers it downstream to a first heat exchanger that, in the case of a cooling mode of operation, rejects heat to the ambient environment. The refrigerant passes from the first heat exchanger to an expansion device, and then through a second heat exchanger that, in the cooling mode of operation, cools a secondary fluid (e.g. air) to be delivered to a conditioned environment. From the second heat exchanger the refrigerant passes back to the compressor.
One known type of an expansion device is an expansion valve. In the expansion valve, a sensor (for an electronic expansion valve) or bulb (for a thermostatic expansion valve) is positioned at a specific location within the refrigerant system. This sensor communicates operating conditions such as a temperature, pressure, superheat or a combination of thereof back to the expansion valve. This feedback serves to adjust (open or close) a variable orifice through the expansion device such that a desired amount of refrigerant is allowed through the expansion device.
While expansion devices are widely utilized, there are some challenges associated with their applications. Such challenges include operation of these devices over a wide spectrum of indoor and outdoor environments as well as a need to handle transient conditions. In some applications, the amount of refrigerant circulating throughout the system can vary by two orders of magnitude depending on indoor and outdoor environments and transient system demands. For instance, the conditions requiring high mass flow of refrigerant to be circulated through the system may occur at a pulldown immediately after the startup, or when hot (and potentially humid) outdoor air is brought in to be conditioned or refrigerated to a desired temperature. On the other hand, part-load conditions at relatively cold ambient temperatures do not require high refrigerant system capacity, and the refrigerant mass flow rate must remain low.
Since, the expansion valve needs to be sized to handle all of the conditions, a relatively large valve would be required. This is unduly expensive and, in some cases, impractical. Moreover, when the refrigerant system is operating at more typical part-load conditions or at very low evaporator temperatures, the oversized expansion valve may not be able to precisely meter the refrigerant to achieve the desired performance characteristics at this part-load operation. Also, the larger size expansion device may not close completely, which can lead to refrigerant leakage at shutdown, or may take a longer time to close allowing more than desirable amount of refrigerant to migrate from high to lower pressure side of the system on a shutdown.